A cell is the basic unit of living organisms that is capable of reproducing itself exactly. That is, although eukaryotic cells (such as plant cells and animal cells) contain components, which are called organelles, those components do not reproduce themselves independently of the cell of which they are a part.
Cell membranes consist of lipids and proteins and are selectively-permeable to allow materials to move both into and outside of the cell. Animal cells have cell membranes - as opposed to the cell walls of plant cells.
The cell membrane keeps the cell together by containing the organelles within it.
See also the functions of cell organelles.